Heat-generating parts such as power transistors and thyristors deteriorate their performance due to the heat generated. It is a common practice in the prior art that such heat-generating parts are provided with heat sinks for heat dissipation or suitable means for conducting heat to a metal chassis of the associated equipment for heat release. To improve both electrical insulation and heat transfer, heat-dissipating, electrically insulating sheets of silicone rubber loaded with heat conductive fillers often intervene between heat-generating parts and heat sinks.
As the heat-dissipating, electrically insulating material, JP-A 47-32400 discloses an electrically insulating composition comprising 100 parts by weight of synthetic rubber, typically silicone rubber and 100 to 800 parts by weight of at least one metal oxide selected from beryllium oxide, aluminum oxide, hydrated aluminum oxide, magnesium oxide, and zinc oxide.
As the heat-dissipating material for use in areas where electrical insulation is not required, JP-A 56-100849 corresponding to U.S. Pat. Nos. 4,292,223, 4,292,224, 4,292,225, and U.S. Pat. No. 4,293,477 discloses a composition comprising 100 parts by weight of an addition curing type silicone rubber and 60 to 500 parts by weight of silica and a heat conductive powder such as silver, gold or silicon.
These heat conductive materials, however, have a thermal conductivity of less than 1.5 W/mK. If silicone rubber compositions are loaded with large amounts of heat conductive fillers in order to improve the heat transfer thereof, the compositions lose fluidity and become very difficult to mold and work.
One solution to this problem is found in JP-A 1-69661 which discloses a good heat conductive rubber/plastic composition loaded with alumina consisting of 10 to 30% by weight of alumina particles having an average particle size of up to 5 μm and the balance of spherical corundum of single particles having an average particle size of at least 10 μm and of cutting edge-free shape. Also, JP-A 4-328163 corresponding to U.S. Pat. No. 5,352,731 discloses a heat conductive silicone rubber composition comprising 100 parts by weight of a base of an organopolysiloxane gum having an average degree of polymerization of 6,000 to 12,000 combined with an organopolysiloxane oil having an average degree of polymerization of 200 to 2,000 and 500 to 1,200 parts by weight of spherical aluminum oxide powder.
However, in the case of heavy loading of more than 1,000 parts by weight of aluminum oxide powder per 100 parts by weight of the base polymer or more than 70% by volume of aluminum oxide, for example, these methods relying merely on a combination of coarse and fine particles or a viscosity adjustment of the silicone base encounter a certain limit in improving the moldability and workability of silicone rubber compositions.
It would be desirable to improve the moldability and workability of silicone rubber compositions. JP-A 2000-256558 corresponding to U.S. Pat. No. 6,306,957 discloses a heat conductive silicone rubber composition comprising 0.1 to 50% by volume of a hydrolyzable radical-containing methylpolysiloxane as a wetter. The use of the wetter is successful in improving the moldability and workability of heat conductive silicone rubber compositions. However, as the proportion of the wetter added is increased for achieving higher heat conduction, the composition is minimized in viscosity buildup, but loses its fluidity.